Particulars of such a machine tool spindle were first published on Apr. 26, 1985 in Frankfurt in a VDMA-Tagung (Documents of the firm VALENITE-MODCO. GmbH, D-6920 Sinheim/Elsenz "VAL-clamping device for flexible production").
Such a known machine tool spindle serves for the selective reception of a tool holder with a conventional steeply tapered conical shank (DIN 2080 or 69871) or of a new kind of tool holder which has a cylindrical mating spigot adjoining a flange. Tool holders which have a steeply tapered conical shank usually also have a flange which is often formed with a circumferential trapezoidal groove for engagement by the changing arm of an automatic tool-changing mechanism. In the case of tool holders with a steeply tapered conical shank, however, the flange usually does not make contact with the end surface of the spindle. On the contrary, the tool holder is supported solely by the engagement of its steeply tapered conical shank in the frusto-conical receiving bore in the spindle. If then the length of this frusto-conical receiving bore is reduced by about half as a result of the provision of the cylindrical mating bore, a large part of the supporting surface between the steeply tapered conical shank and the frusto-conical receiving bore will be lost. Consequently, the connection between the tool holder and the spindle which in any case fails to satisfy modern requirements in regard to power transmission is still further seriously impaired. The centering action between the steeply tapered conical shank and the frusto-conical receiving bore is also insufficient in relation to contemporary accuracy requirements. If, however, a tool holder with a cylindrical mating extension is inserted in the spindle, then a better centering between the tool holder and the machine tool spindle will be obtained, but at the same time the supporting of the said tool holder transversely to the axis of the spindle will no longer be so good since the cylindrical mating bore commences at a greater distance from the end surface of the spindle. Furthermore, the known tool holder with a cylindrical mating extension has two diamterically oppositely located clamping members which are mounted for radial sliding movement with respect to the axis of the spindle. Between the inner ends of these clamping members an axially movable wedge is provided which is connected to a draw bolt projecting outwardly from the frusto-conical part. The draw bolt is engaged by the collet provided on the pull rod which is slidably mounted in the spindle of the machine tool. As a result of the axial movement of the said spindle, the clamping members can be forced radially outwardly so that inclinded surfaces provided on their outer ends engage in recesses formed in the wall of the mating bore. The flange is consequently pressed against the end surface of the spindle. A disadvantage of this arrangement is that no particularly high degree of accuracy is obtained in the repetitive work done during the changing of the tool holders. This can be attributed to the fact that, in spite of the close tolerances between the mating extension and the mating bore, some play must nevertheless still be present between these two parts. Since the two clamping members on the radially opposite sides of the mating extension are forced outwardly, approximately equally large reaction forces act on the mating extension in opposite directions. Depending upon the frictional relationships between the inclined surfaces, the mating spigot will be so oriented with respect of the mating bore that it will bear against the wall of the mating bore either in the vicinity of one of the clamping members or in the vicinity of the oppositely located clamping member. It may also happen, however, that the mating extension assumes some intermediate position between two limiting positions. This impairs the interchange tolerance and hence the accuracy with which operations are repeated during frequent interchanging of identical tool holders or of dissimilar tool holders to quite a substantial extent.
The problem underlying the invention is to improve a machine tool spindle and tool holders suitable therefor which have different shanks of the kind initially referred to in such a manner that, when a tool holder having a steeply tapered conical shank is inserted, the connection between it and the machine tool spindle is impaired to the smallest possible extent and that, when a tool holder having a cylindrical extension is inserted, an optimum transverse support with very great accuracy of centring is attained. Furthermore, the accuracy achieved in operations which are repeated during the changing of the tool holders will be improved.
The solution of this problem consists according to the invention in that the mating bore directly adjoins the end surface of the spindle and the diameter thereof is slightly greater than the theoretical diameter of the frusto-conical receiving bore in the plane of the end surface and that the axial length of the mating bore is equal to only about 10 to 15% of the theoretical diameter.
Since the mating bore has only a relatively short axial length, only a relatively small part of the frusto-conical receiving bore is lost by the provision of the mating bore. During the insertion of a tool holder having a steeply tapered conical shank, a relatively large supporting surface is available between the said steeply tapered conical shank and the frusto-conical receiving bore, despite the presence of the cylindrical mating bore, so that the connection between the steeply tapered conical shank and the machine tool spindle is scarcely impaired at all by the provision of the mating bore. The arrangement of the mating bore directly adjacent the end surface of the machine tool spindle has the advantage that, during the insertion of tool holders having a cylindrical mating extension, the transverse support of the tool holder takes place in the immediate vicinity of the end surface. Forces acting transversely to the axis of the spindle are consequently transmitted in a most favourable manner from the tool holder to the spindle and vice versa. This is of special importance in the case of tool holders with a cylindrical mating extension, since these tool holders, because of the wide area support of their flanges against the end surface of the machine tool spindle, are particularly suitable for the transmission of relatively large working forces. Furthermore, it is also possible to keep the tolerance between the cylindrical mating extension and the cylindrical mating bore particularly close, since because of the short axial lengths of both parts they must also be displaced axially through only a very small distance with respect to each other.
A particularly advantageous arrangement of the invention consists in the fact that a clamping member is arranged for sliding movement in a radial guideway provided in the frusto-conical member in the vicinity of the mating extension, which clamping member is radially movable by means of a wedge surface engaging its inner end and connected with the draw-in bolt arranged for axial sliding movement in the frusto-conical part and is provided at its outer end with a thrust surface which can be pressed against a counter-surface provided on the spindle.
The use of only one clamping member has the advantage that, during the outward displacement of the clamping member and the pressing thereof against the counter-surface, a reaction force is produced on the cylindrical mating extension which brings the latter into contact with the wall of the mating bore on the side thereof diameterically opposite the clamping member. The tool holder is thereby automatically brought to bear, during each tool change, in a clearly defined manner against the same side of the mating bore. By this means very small interchanging tolerances are obtained with a high degree of accuracy of repetition.